1. Field of the Invention
The present invention relates to a passive infrared ray sensor for detecting an intruder upon receipt of infrared rays of light emanating from a person intruding into a detection region.
2. Description of the Prior Art
In this type of the passive infrared ray sensor, infrared rays of light emanating from an intruder then entering in a detection region is collected by an optical element and then received by a detection element such as, for example, a pyroelectric element and a signal detecting process such as, for example, analysis of the waveform of a detection signal from the pyroelectric element, calculation and detection determination is carried out by a microcomputer (a detection processing unit) to thereby detect an intruder.
Since a signal detecting process performed by the microcomputer is generally operated with a rated electric current, the amount of an electric power consumed tends to be large and, hence, there has been such a problem that in the case of a detection sensor of a battery driven type, the lifetime of the battery tends to be reduced. Accordingly, the JP Laid-open Patent Publication No. 2002-156281, first published May 31, 2002, for example, discloses the use of a mode switching means for selecting one of an operating mode, in which the microcomputer performs the signal detecting process, and a standby mode, in which no signal detecting process is performed, such that during the stand-by mode the electric current is limited to a value lower than the rated electric current, in the event that a detection signal is lower than a activation threshold value which defines a standard at which the signal detecting process is initiated, but in the event that the detection signal is not lower than a activation threshold value, the microcomputer is operated under the operating mode.
In the meantime, in the case of the detection sensor of a type using two pyroelectric elements and operable to detect an intruder when signal levels of those detection signals from those pyroelectric elements are not all lower than a determination threshold value defining the reference for the intruder detection, when the detection signals of those two pyroelectric elements are stabilized at respective values lower than the activation threshold value, the microcomputer is set under the standby mode to thereby suppress the power consumption. In the event that the detection signal of either one of the pyroelectric elements is not lower than the activation threshold value, the microcomputer is set under the operating mode to analyze the detection signals of those pyroelectric elements so as to determine whether or not the detection signals of those two pyroelectric elements are not lower than the determination threshold value, thereby performing the signal detecting process.
In the case of the detection sensor utilizing the two pyroelectric elements, however, if the activation threshold value is lowered, in the event that in one of the pyroelectric elements, fluctuation of the detection signal occurs so frequently as to result in an instable condition because of noises induced by external environments such as, for example, trees or grasses within the detection area are swayed by the wind or heats of the sun or small animals run in and out, the signal from the pyroelectric element is analyzed each time the instable condition occurs enough to set the microcomputer under the operating mode and, therefore, reduction in electric power consumption cannot be achieved under the external environment full of noises, accompanied by reduction in battery lifetime. On the other hand, if the activation threshold value is increased, the difference between the activation threshold value and the determination threshold value is reduced and, depending on the detection signal, the waveform analysis may take place only with the waveform of peaks of the detection signal exceeding the activation threshold value and, therefore, no accurate signal pattern analysis may be carried out, accompanied by an erroneous recognition (erroneous warning) and/or a missing (failure to warn) of an intruder because of unavailability of the detection signal of a length required for the determination.